Measuring your knitting in progress may seem like a no-brainer, but asking how to measure properly is a very good question. Firstly, please use a flexible, undamaged tape measure.
To measure knitting length (ribbing, garment piece, or sleeve), lay the piece on a flat surface; a low pile carpet, ironing board or blocking board. Don’t place on a smooth surface such as wood table top, and definitely not on your lap; both of these surfaces will give inaccurate measurements. Without stretching the piece, smooth the surface of the knitting, trying to evenly space the stitches around the center area, particularly if there lots of stitches. It is easy to do this with a piece like in the image below, a baby sweater sleeve. A ribbed pattern is a little tricky to measure, because of its elasticity; stretch a little to reveal pattern, unless instructions indicate otherwise. Place the tape measure at the center of the work, from the base of the knitting needle to the bottom edge.
To measure a shaped area such as an armhole, measure perpendicular to the bottom edge, lining the tape from the base of the needle to the first bound off row of the armhole. When measuring the length of the sleeve, always measure from the center at the base of the needle to the lower edge. Do not follow along any slanted or shaped edge, as this will result in an erroneous measurement – armholes that are too shallow, and sleeves too short.
Just a small tip; I like to write down my border measurements, and then I subtract this from the total piece measurement, in order to figure out row counts. Another reason for recording this measurement is because borders are often done on smaller needles, so the length of the border will be slightly smaller than the gauge for the body of the piece.
Hopefully this helps beginners to measure more accurately.